This invention relates to gamma cameras, and more particularly to a gamma camera capable of producing reasonable images of dynamic body functions.
Conventional gamma cameras of the type disclosed in Anger U.S. Pat. No. 3,011,057 typically operate at count rates of about 100,000 counts per second. Using radioisotopes producing a flux with this count rate, the frame time sufficient to accumulate data to construct an image is of such duration that only limited dynamic processes can be investigated. In other words, a highly dynamic function such as regional cerebral blood flow would require a relatively short-lived isotope producing a relatively high flux of radiation of about 500,000 counts per second; and such a function cannot be imaged with a conventional gamma camera. While multi-crystal gamma cameras are known with the potential for imaging highly dynamic processes, such a camera, by reason of its multiplicity of crystals, is not only extremely complex and hence more expensive to build, operate and maintain than the relatively simple single crystal Anger type gamma camera, but has reduced resolution.
At the present time, the upper limit on the counting rate is about 200,000 counts per second which produces a 1.5 usec dead time, i.e., the time following a scintillation in the crystal during which the camera is blocked and prevented from processing a subsequent event. Such dead time has two components: the decay time of a scintillation that results when a radiation stimulus interacts with the crystal of the camera, and the computation or recovery time of the electronics associated with the computation circuitry by which signals representing the coordinates of a scintillation are computed. As to the event itself, it is approximated by the relationship (I) exp (-t/.tau.), where I is the amplitude of the pulse, t is time and .tau. is the time constant associated with the crystal. For a conventional sodium iodide thallium-activated crystal, the time constant is about 220 nsec which means that about one .mu.sec (i.e., about five time constants) is required to collect 99 percent of the light produced by an event.
In the usual gamma camera, computation begins after the termination of the scintillation or the scintillation with the result that no event occurring during the relatively long period comprehending the decay time of the scintillation and computation can be accepted by the camera. Where two events occur sequentially within a time less than the order of about 5.tau. the resulting "pile-up", if not compensated for, will produce erroneous results that further degrade image quality.
It is therefore an object of the present invention to provide a new and improved gamma camera in which dead time is significantly reduced as compared to the prior art while still enabling images of acceptable quality to be produced for high count rate imaging.